DESCRIPTION

These functions provide a simple interface for asynchronous execution of
code.
The function taskqueue_create() is used to create new queues. The
arguments to taskqueue_create() include a name that should be unique, a
set of malloc(9) flags that specify whether the call to malloc() is
allowed to sleep, a function that is called from taskqueue_enqueue() when
a task is added to the queue, and a pointer to the memory location where
the identity of the thread that services the queue is recorded. The
function called from taskqueue_enqueue() must arrange for the queue to be
processed (for instance by scheduling a software interrupt or waking a
kernel thread). The memory location where the thread identity is
recorded is used to signal the service thread(s) to terminate--when this
value is set to zero and the thread is signaled it will terminate. If
the queue is intended for use in fast interrupt handlers
taskqueue_create_fast() should be used in place of taskqueue_create().
The function taskqueue_free() should be used to free the memory used by
the queue. Any tasks that are on the queue will be executed at this time
after which the thread servicing the queue will be signaled that it
should exit.
To add a task to the list of tasks queued on a taskqueue, call
taskqueue_enqueue() with pointers to the queue and task. If the task’s
ta_pending field is non-zero, then it is simply incremented to reflect
the number of times the task was enqueued. Otherwise, the task is added
to the list before the first task which has a lower ta_priority value or
at the end of the list if no tasks have a lower priority. Enqueueing a
task does not perform any memory allocation which makes it suitable for
calling from an interrupt handler. This function will return EPIPE if
the queue is being freed.
The function taskqueue_enqueue_fast() should be used in place of
taskqueue_enqueue() when the enqueuing must happen from a fast interrupt
handler. This method uses spin locks to avoid the possibility of
sleeping in the fast interrupt context.
To execute all the tasks on a queue, call taskqueue_run() or
taskqueue_run_fast() depending on the flavour of the queue. When a task
is executed, first it is removed from the queue, the value of ta_pending
is recorded and then the field is zeroed. The function ta_func from the
task structure is called with the value of the field ta_context as its
first argument and the value of ta_pending as its second argument. After
the function ta_func returns, wakeup(9) is called on the task pointer
passed to taskqueue_enqueue().
The taskqueue_drain() function is used to wait for the task to finish.
There is no guarantee that the task will not be enqueued after call to
taskqueue_drain().
The taskqueue_member() function returns 1 if the given thread td is part
of the given taskqeueue queue and 0 otherwise.
A convenience macro, TASK_INIT(task, priority, func, context) is provided
to initialise a task structure. The values of priority, func, and
context are simply copied into the task structure fields and the
ta_pending field is cleared.
Five macros TASKQUEUE_DECLARE(name), TASKQUEUE_DEFINE(name, enqueue,
context, init), TASKQUEUE_FAST_DEFINE(name, enqueue, context, init), and
TASKQUEUE_DEFINE_THREAD(name) TASKQUEUE_FAST_DEFINE_THREAD(name) are used
to declare a reference to a global queue, to define the implementation of
the queue, and declare a queue that uses its own thread. The
TASKQUEUE_DEFINE() macro arranges to call taskqueue_create() with the
values of its name, enqueue and context arguments during system
initialisation. After calling taskqueue_create(), the init argument to
the macro is executed as a C statement, allowing any further
initialisation to be performed (such as registering an interrupt handler
etc.)
The TASKQUEUE_DEFINE_THREAD() macro defines a new taskqueue with its own
kernel thread to serve tasks. The variable structtaskqueue*taskqueue_name is used to enqueue tasks onto the queue.
TASKQUEUE_FAST_DEFINE() and TASKQUEUE_FAST_DEFINE_THREAD() act just like
TASKQUEUE_DEFINE() and TASKQUEUE_DEFINE_THREAD() respectively but
taskqueue is created with taskqueue_create_fast().
PredefinedTaskQueues
The system provides four global taskqueues, taskqueue_fast,
taskqueue_swi, taskqueue_swi_giant, and taskqueue_thread. The
taskqueue_fast queue is for swi handlers dispatched from fast interrupt
handlers, where sleep mutexes cannot be used. The swi taskqueues are run
via a software interrupt mechanism. The taskqueue_swi queue runs without
the protection of the Giant kernel lock, and the taskqueue_swi_giant
queue runs with the protection of the Giant kernel lock. The thread
taskqueue taskqueue_thread runs in a kernel thread context, and tasks run
from this thread do not run under the Giant kernel lock. If the caller
wants to run under Giant, he should explicitly acquire and release Giant
in his taskqueue handler routine.
To use these queues, call taskqueue_enqueue() with the value of the
global taskqueue variable for the queue you wish to use (taskqueue_swi,
taskqueue_swi_giant, or taskqueue_thread). Use taskqueue_enqueue_fast()
for the global taskqueue variable taskqueue_fast.
The software interrupt queues can be used, for instance, for implementing
interrupt handlers which must perform a significant amount of processing
in the handler. The hardware interrupt handler would perform minimal
processing of the interrupt and then enqueue a task to finish the work.
This reduces to a minimum the amount of time spent with interrupts
disabled.
The thread queue can be used, for instance, by interrupt level routines
that need to call kernel functions that do things that can only be done
from a thread context. (e.g., call malloc with the M_WAITOK flag.)
Note that tasks queued on shared taskqueues such as taskqueue_swi may be
delayed an indeterminate amount of time before execution. If queueing
delays cannot be tolerated then a private taskqueue should be created
with a dedicated processing thread.